1. Field of the Invention
The present invention is related to the use of goniospectrophotometric, goniospectroradiometric, goniophotometric, or gonioradiometric measurements of primary or secondary sources, including the fields of color measurement, scatterometry, multi-angle light scattering (MALS), and beam profiling.
2. Discussion of Related Art
Goniospectrophotometers are used for characterizing the spectra of light as a function of angle for light sources such as LEDs, and also for materials painted or printed with gonioapparent or pearlescent pigments in the paint and ink. Conventional commercially-available systems obtain the goniospectra either by using an optical fiber coupled spectrophotometer and moving the fiber about the source or sample, or by using a set of spectrometers positioned at a set of fixed angles. Measurement times for multiple zenith angles at a single azimuth angle about the source are typically in the range of 1-20 minutes.
Conventional practice uses mechanical scanning of a detection system (such as a fiber-coupled spectrograph) about the source. FIG. 1 shows a schematic diagram of a conventional Goniospectrophotometer 100. Detection system 103 of goniospectrophotometer 100 can, for example, be a fiber-optic cable attached to a spectrophotometer to provide spectral data regarding light source 101. As such, detector system 103 is highly directional in its receipt of optical radiation from source 101 and therefore can be configured to receive light at substantially a single zenith angle at a single azimuth angle from light source 101. The distance between detection system 103 and light source 101 is typically within the “near-field” region, but may also be in the “far-field” region. The “far-field” region is characterized by a distance far enough from light source 101 such that light source 101 can be viewed as a point source. A commonly used rule of thumb boundary between the two regions is the “Five-Times Rule,” where detection system 103 is placed at a distance from light source 101 that is at least five times the lateral extent of light source 101.
In operation, detector system 103 is scanned around light source 101 stopping at each of a set of predetermined zenith angles to measure a spectrum with a spectrometer. Thus, a spectral scan as a function of angular position for a single azimuth angle around light source 101 is obtained. In some embodiments, detector system 103 can be rotated along a great circle on a sphere with light source 101 at its center. Detector system 103 can also be oriented so that it collects light in a small solid angle substantially originating from the center of the sphere.
FIG. 2 illustrates an example where source 101 is illuminated by light beam 105. Light beam 107 is a reflection from light source 101 of illuminating beam 105. As shown in FIG. 2, light source 101 may be a material deposited on a base or substrate 109. In some testing situations, scatter of light beam 105 is measured in detector system 103. In some testing situations, light from light source 101 is a result of luminescence after excitation by illuminating light beam 105.
Performing a spectral scan at each angular position, however, takes a significant amount of time to perform. Therefore, systems such as those shown in FIG. 1 and 2 can be very slow. As mentioned above, a spectral scan at an individual azimuth can take up to about 20 minutes. Therefore, an angular distribution of that scan can involve a few hundred to a few thousand minutes (depending on the number of angles) to execute.
FIG. 3 illustrates a goniospectrophotometer 300 that mitigates the scanning time somewhat. As shown in FIG. 3, goniospectrophotometer 300 includes a plurality of detector systems 301-1 through 301-N. Detector systems 301-1 through 301-N are fixed at particular angles around light source 101 so that each receives light from light source 101 at a particular zenith angle θ1 through θN. As described above, each of detector systems 301-1 through 301-N can include an optical fiber directed to receive light at an azimuthal angle θ1 through θN from light source 101. Each of detector systems 301-1 through 301-N can include a spectrometer for providing a spectrum at the corresponding one of zenith angles θ1 through θN.
FIG. 4 illustrates a goniospectrophotometer 300 utilized to characterize a light source 101 illuminated by beam 105. Light source 101, which can be on substrate 109, reflects, luminesces, or otherwise emits light in response to illumination beam 105. Illumination beam 105 is reflected from light source 101 into reflection beam 107.
Although the embodiments of goniospectrophotometer 300 shown in FIGS. 3 and 4 reduce the amount of time required to provide a spectral scan of light source 101, they are expensive in that each of detector systems 301-1 through 301-N is an independent detector system that requires an independent spectrometer. Further, goniospectrophotometer 300 suffers from the need to calibrate multiple detector systems 301-1 through 301-N. Also, there may be difficulty in measuring continuous angular spectra due to mechanical obstructions from mounts utilized to hold other detector systems 301-1 through 301-N in place.
Therefore, there is a need for economical, faster methods of obtaining the angular dependence of color spectra of light from material samples or light sources for applications requiring goniospectrophotometric analyses.